Vehicles have been designed to consume less fuel by decreasing the rolling resistance of tires so as to suppress heat build-up. These days, the demand for tires with higher fuel economy has been increasing. Accordingly, fuel economy (lower heat build-up) is required not only for treads and sidewalls but also for other tire components such as undertreads.
Examples of known methods for reducing the heat build-up of rubber compositions for undertreads include a method using semi-reinforcing filler, a method involving reducing the amount of filler, and a method using silica as filler. However, these methods reduce the reinforcement of the rubber compositions, thereby leading to a decrease in tensile properties. Thus, it has been difficult to improve fuel economy and tensile properties at the same time.
Meanwhile, natural rubber is commonly used for undertreads. Natural rubber has a higher Mooney viscosity than other synthetic rubbers and thus has poor processability. Therefore, natural rubber is usually mixed with a peptizer and masticated so as to reduce the Mooney viscosity before use. Hence, the productivity is low when natural rubber is used. Further, mastication causes molecular chain scission of natural rubber, which disadvantageously leads to a loss of intrinsic properties (e.g. fuel economy and rubber strength) of the high-molecular-weight polymer.
Although Patent Literature 1 suggests the use of natural rubber that is deproteinized such that the total nitrogen content falls to 0.1% by weight or less, there is still room for improvement in improving processability, fuel economy, and tensile properties in a balanced manner.